This is actually very old bug, but it started to actively show itself only in 2.5 - most possible due to more places when threads can run together in the engine. I will describe it on a simple and not widely used today sample, but it obviously can happen in other cases too.
Imagine some system relation (for example rdb$database) is modified in user transaction (this really happens in GPRE-preprocessed programs to set database properties). Transaction is committed, and there is an old version of the record on the disk. Right after it CREATE TABLE is issued, which modifies same relation rdb$database in system transaction. System transaction always uses update_in_placed() to update a record. But during it another thread (on SS this is GC thread, but it can easily be any regular attachment reading rdb$database) removes old unused version of the record. At this moment system transaction fails with bugcheck(291) - no record version found.